Aortic Dissection

Diagnostics and Likelihood Ratios, Explained

Positive Findings (Patient Has This)

SymptomsIncreased Disease Probability (Positive Likelihood Ratio)
Tearing/Ripping Pain10.8× (5.2-22.0)
Migrating pain7.6× (3.6-16.0)
Sudden Chest Pain2.6× (2.0-3.5)
Hx of Hypertension1.5× (0.8-3.0)
Signs on Physical ExamIncreased Disease Probability (Positive Likelihood Ratio)
Focal Neurologic Deficit33.0× (2.0-549.0)
Diastolic Heart Murmur4.9× (0.6-40.0)
Pulse Deficit2.7× (0.7-9.8)
Lab and Study FindingsIncreased Disease Probability (Positive Likelihood Ratio)
Enlarged Aorta or Wide Mediastinum3.4× (2.4-4.8)
LVH on Admission EKG3.2× (1.5-6.8)

Negative Findings (Patient Doesn't Have This)

SymptomsDecreased Disease Probability (Negative Likelihood Ratio)
Sudden Chest Pain0.3× (0.2-0.4)
History of Hypertension0.4× (0.3-0.6)
Tearing/Ripping Pain0.4× (0.3-0.5)
Migrating Pain0.6× (0.5-0.7)
Signs on Physical ExamDecreased Disease Probability (Negative Likelihood Ratio)
Pulse deficit0.63× (0.4-1.0)
Focal Neuro deficit0.87× (0.8-0.9)
Diastolic murmur1.1× (0.6-1.7)
Lab and Study FindingsDecreased Disease Probability (Negative Likelihood Ratio)
Enlarge aorta or wide mediastinum0.13× (0.02-1.00)*
LVH on admission EKG0.84× (0.7-0.9)


  • Outcome: Thoracic Aortic Dissection
  • Multiple types of studies were reviewed in this RCE.
  • This spreadsheet reviews prospective studies only.
  • LR's are listed for these prospective studies.


Other studies were omitted due to methodological limitations.


Rodrigo Kong, MD


February 22, 2012

What are Likelihood Ratios?

LR, pretest probability and posttest (or posterior) probability are daunting terms that describe simple concepts that we all intuitively understand.

Let's start with pretest probability: that's just a fancy term for my initial impression, before we perform whatever test it is that we're going to use.

For example, a patient with prior stents comes in sweating and clutching his chest in agony, I have a pretty high suspicion that he's having an MI – let's say, 60%. That is my pretest probability.

He immediately gets an ECG (known here as the "test") showing an obvious STEMI.

Now, I know there are some STEMI mimics, so I'm not quite 100%, but based on my experience I'm 99.5% sure that he's having an MI right now. This is my posttest probability - the new impression I have that the patient has the disease after we did our test.

And likelihood ration? That's just the name for the statistical tool that converted the pretest probability to the posttest probability - it's just a mathematical description of the strength of that test.

Using an online calculator, that means the LR+ that got me from 60% to 99.5% is 145, which is about as high an LR you can get (and the actual LR for an emergency physician who thinks an ECG shows an obvious STEMI).

(Thank you to Seth Trueger, MD for this explanation!)